As graphite particles that are used as a negative electrode material, for instance, natural graphite particles, artificial graphite particles obtained by graphitizing cokes, artificial graphite particles obtained by graphitizing organic polymers, pitch or the like, graphite particles obtained by pulverizing these and so on can be cited. These particles are mixed with an organic binder and a solvent to form a graphite paste, followed by coating the graphite paste on a surface of a copper foil, further followed by drying the solvent, and used as a negative electrode of a lithium ion secondary battery. For instance, as shown in Japanese Patent Application Publication No. 62-23433, by use of graphite in a negative electrode, a problem of the internal short-circuiting owing to the dendrite of lithium is eliminated and thereby the cycle characteristics are improved.
However, in natural graphite particles in which graphite crystal are developed and artificial graphite particles obtained by graphitizing cokes, since a binding force between crystal layers in a c-axis direction is weaker than that in an in-plane direction in a crystal, bonds between graphite layers are disbanded owing to pulverization, resulting in so-called scaly graphite particles having large aspect ratio. Since the scaly graphite particles have large aspect ratio, when they are kneaded with a binder and coated on a current collector to prepare an electrode, the scaly graphite particles are aligned in an in-plane direction of the current collector. As a result, because of the distortion generated in a c-axis direction owing to repetition of storage and release of lithium to the graphite particles, the inside of the electrode is destroyed, resulting in not only deteriorating the cycle characteristics but also tending to deteriorate the discharge load characteristics. Furthermore, since the specific surface area of the scaly graphite particles having large aspect ratio is large, the adhesiveness with the current collector is poor; accordingly, unfavorably large amount of binder is necessary. The poor adhesiveness with the current collector causes a problem of deteriorating the current collecting property, resulting in deteriorating discharge capacity, discharge load characteristics and cycle characteristics. Still furthermore, there is a problem that the irreversible capacity at the first cycle of a lithium ion secondary battery using the scaly graphite particles having large specific surface area is large. Furthermore, the scaly graphite particles having large specific surface area exhibit a low thermal stability when lithium is stored, therefore there is a problem in the safety when it is used in a negative electrode material of a lithium ion secondary battery. Accordingly, graphite particles that allow improve in discharge load characteristics, cycle characteristics and irreversible capacity at the first cycle are in demand.
To satisfy the above demands, graphite particles where a plurality of flat particles are assembled or bonded so that a plurality of alignment surfaces may be non-parallel with each other (hereinafter referred to as non-alignment graphite particles) are proposed (for instance, Japanese Patent Application Laid-Open No. 10-158005). A lithium ion secondary battery using the non-alignment graphite particles as a negative electrode material has high discharge capacity and has excellent discharge load characteristics, the cycle characteristics and the charge/discharge efficiency at the first cycle. Accordingly, it can be preferably used in a lithium ion secondary battery. However, there is a problem that the charging capacity (charge load characteristics) when it is charged at a high-speed is low.